Microphone preamplifier



United States Patent Ofilice dg Patented Aug. '7, 1962 z claims. (ci.riai) The present invention relates to transistor amplifiers, Iand moreparticularly to a transistor amplifier that is especially suited as apreamplier `for `a dynamic microphone. This application is acontinuation of our cepending application Serial No. 472,672, filedDecember 2, `1954, now abandoned.

The need has existed for an amplifier that could `be used with a dynamictype microphone so that the advantages of high iidelity and low noiseinherent in this type of microphone could be combined with the advantageof high output such as obtained from a carbon microphone. It is mostdesirable that such an amplifier be small, simple and easily powered sothat it may be built into 4the casing of the microphone itself and thusreduce the effects of hum pickup that occurs when the microphone leadshave any appreciable length. The advent of the transistor made this typeof :amplifier possible.

It is, accordingly, :an object of the present invention to provide animproved transistor amplifier of the above described type that issuitable for use as a dynamic microphone preamplifier.

Another `object of the invention is to provide such an improvedtransistor amplifier that is eminently simple and compact in its designso that it may be incorporated directly in the casing of the microphone.

A `:feature of the invention is the provision of a transistor amplifierthat incorporates but a single transistor stage, and which utilizes asingle pair of leads to serve the dual purpose of :receiving `the DC.power for the .amplifier and of providing the audio output. Thisconstruction not only enables the amplifier to be conveniently mountedin the microphone casing without increasing its size unduly, but alsokeeps the number of leads extending from the microphone-amplifierassembly to a minimum.

The a-bove and other features of the invention which are believed to benew are set forth with particularity in the appended claims. Theinvention itself, however, together with further objects and advantagesthereof may best be understood by reference to the following descrip-:tion when taken in conjunction With the accompanying drawing, in which:

FIG. 1 shows one embodiment of the transistor microphone preamplifier ofthe present invention; and

FIGS. 2 and 3 show la modification of the circuit.

The invention provides an amplifier including in combination atransistor having a base electrode, an emitter electrode and a collectorelectrode. An input circuit is provided which includes the inductancecoil of a microphone, with one side of the induotance coil beingconnected to the base electrode. First resistor means connects the otherside of the inductance coil to a iirst output terminal, and second andthird resistance means are ser-ies connected yfrom the junction of thecoil and the first resistor means to the emitter electrode. The secondand third resistor means are by-passed respectively by first and secondcapacitor means. A connection extends from the junction of the secondand third resistor means to a second output terminal, and anotherconnection extends from the collector electrode to the first outputterminal.

A pair of input terminals are provided which are adapted to receive theoutput terminals referred to above.

A direct current load impedance element connects one of the inputterminals to the positive terminal of a source of unidirectionalpotential, and a connection extends from the first input terminal to thenegative terminal of the source. Capacitor means is connected across thepositive and negative terminals to `by-pass the source, and a loadoutput terminal is connected to the load impedance to derive an outputsignal from the amplifier.

The circuit of FIG. 1 includes a dynamic microphone 10 having aninductance winding 11. One side of the winding is connected to the baseelectrode 12 of a transistor 13 land the other side is connected to anoutput terminal 14 through a resistor 1S. The junction `of winding 11and resistor 15 is connected through a resistor 16 to a second outputterminal 17, and resistor 16 is shunted by a capacitor 1'8. Terminal 14is directly connected to the collector electrode 19 of transistor 13,and terminal 17 is connected through a resistor 20 to the emitterelectrode 21 of the transistor, resistor 2t) being shunted by acapacitor 22.

Terminals 14 and `17 represent the output terminals of thetransistor-amplifier combination, it being understood that thecombination may be mounted Within a single casing. Terminals 14 and 17are connected through a two lead cable to the audio input circuit ofthetransmitter, public address system, etc., with which `the assembly is tobe associated by way of input terminals 25 and 26. Input terminal 2Sreceives the terminal 17 and input terminal Z6 receives terminal 14.Terminal 26 is connected directly to ground, and terminal 25 isconnected through a load resistor Z7 to the positive terminal 28 of adirect current source of, for example, six volts. The negative terminal29 of the source is connected directly to ground, and the source isby-passed by a capacitor 3G. Load output terminal 31 is connected to thejunction of terminal 2S and load resistor 27, and a second load outputterminal 32 is connected to the grounded negative terminal of the D C.source. Load resistor 27 may be replaced by an output transformer whenso desired or other suitable impedance element.

The amplifier is connected as 1a common emitter type of circuit. Theinput signal from microphone i@ is impressed through capacitors 18 and22 between the emitter and base electrodes, and the output is derivedacross resistor 27 through capacitors 22 and 30 from between thecollector and emitter electrodes. That is, the load resistor 27 is inseries with the emitter `and collector electrodes, and the emitterelectrode is common to the input and output circuits.

Resistors 15, 16 and 20 are for biasing purposes. Resistors 15 and 16provide a relatively fixed small negative voltage on the hase 12 withrespect to the emitter Z1, which causes emitter current to fiow.Resistor 20 reduces the emitter current and tends to oppose any increasetherein so as to control this current in a manner similar to that of acathode resistor in a vacuum tube circuit. This type of self-biasingsystem tends to reduce the effects of different transistorcharacteristics and also the effects of temperature and supply voltagechanges.

Resistor 2? is by-passed `by capacitor 22 to overcome any `degenerativeeffect due to this resistor. Resistor 16 in series with the amplifierinput circuit is relatively small and causes only a small loss in gain,so that its bjr-passing capacitor 1S is not absolutely necessary. It canbe seen that the D.C. power is supplied to the transistor circuit andthe A.C. output is supplied to terminals 311, 32 over but a single pairof leads, and also that the amplifier is extremely simple in itsconstruction and is suitable for mounting in a compact microphonecasing.

The circuit of FIG. 2 is similar to that of FIG. l and like referencenumerals have been used. The circuit of 3 FIG. 2 is intended to be usedwith a D C. source in which the positive terminal is grounded. For sucha connection, the load resistor 27 is included in the negative lead fromterminal 26 as shown.

In a constructed embodiment of the invention, the following constantsWere used, and these are listed herein merely by way of example, and arenot intended to limit the invention in any way:

Capacitor 18 (6` volt tantalum capacitor) rnicrofarads- 25 Resistor 20ohms 10 Resistor 16 do 220 Resistor 1S do 10,000 Resistor 27 do 560 Theembodiments of FIGS. l and 2 can be still further simplified, as shownby the circuit of FIG. 3. The latter circuit has less stabilization thanthe former but is still satisfactory for many applications. In thearrangement of FIG. 3, the stabilizing resistor is removed from theemitter connection and the `forced D C. bias its removed from the base.

The invention provides, therefore, an exceedingly simple transistoramplifier that is eminently suitable for use as a preamplifier for adynamic microphone. The amplifier uses a minimum of components andsimplified circuitry which enables it to `be vconveniently mountedWithin a relatively small microphone casing, and it uses but a singlepair of leads for connecting the microphone-amplifier combination to theinput circuit of the transmitter proper.

What is claimed is:

1. An amplifier and microphone system including in combination, amicrophone with a casing therefor, first amplifier circuit meansincluded Within said casing and having a transistor with base, emitterand collector electrodes, a cable connected to said first circuit meansincluding first and second conductors, said first conductor beingdirectly connected to said collector electrode, direct currentconducting means connecting said second vconductor to said emitterelectrode, a voltage divider comprising first and second resistor meansdirectly connected between first and second conductors, means connectingsaid microphone bet-Ween the junction of said resistor means and saidbase electrode for applying signals to said transistor and biasing saidbase electrode, second amplifier circuit means external to, and remotefrom, said casing of said microphone and including interconnectionthereto by said cable, said second amplifier circuit means including anoutput load impedance for said first amplifier circuit means comprisingthe sole load impedance therefor, said second amplier circuit meansfurther including an audio signal bypass capacitor series connected withsaid output load impedance across said first and second conductors, anddirect current power supply means connected across said bypasscapacitor, whereby quiescent energization for, and output signalcoupling from, said first amplifier circuit means is provided solely bysaid first and second conductors.

2. An amplifier and microphone system including in combination, adynamic microphone having an inductive winding and an enclosing casing,first amplifier circuit means included within said casing and having atransistor with base, emitter and collector electrodes, first capacitormeans coupling said microphone between said base and emitter electrodes,a cable coupled to said system and including a first cable conductordirectly connected to said collector electrode and establishing a groundreference for said system, an emitter bias resistor, said cableincluding a second cable conductor connected through said bias resistorto said emitter electrode, a voltage divider connected between saidfirst and second cable conductors and having an intermediate tapconnected through said dynamic microphone to said base electrode toprovide bias therefor, second amplifier circuit means external to,

and remote from, said casing of said dynamic microphone and includinginterconnection thereto by said first and second cable conductors, saidsecond amplier circuit means including a resistive output load impedancefor said first amplifier circuit means and second capacitor means seriesconnected across said cable conductors with said second capacitor meansconnected to said first cable conductor, and `direct current powersupply means connected across said second capacitor means wherebyquiescent energization for, and output signal coupling from, said firstamplifier circuit means is provided solely by said first and secondcable conductors.

References Cited in the file of this patent UNITED STATES PATENTS2,762,870 Sziklai et al. Sept. Il, 1956 2,774,826 Moulon Dec. 18, 19562,784,262 Crow Mar. 5, 1957 2,842,623 Lehr c Iuly 8, 1958 2,929,877Irick Mar. 22, 1960 FOREIGN PATENTS 862,917 Germany -c Jan. l5, 1953

